The reactivity of hydroxyl radical (OH) with total volatile organic compounds (VOC), &Sgr;kVOC<VOC>, is directly obtained in the planetary boundary layer (PBL). The method is based on a perturbation technique where high initial OH concentrations are created by flash photolysis of ozone and subsequent reaction of O(1D) with H2O. Laser-induced fluorescence is used to measure the residual OH concentration at different time delays after the perturbation (<100 ms) and obtain a direct estimate of the OH lifetime &tgr;OH for different atmospheric conditions. For specific experimental conditions the transport by the wind may be neglected, and the chemical processes governing the OH decay may be expressed by a detailed box model. With a simple chemical equation derived in this paper using &tgr;OH and complementary measurements of CO, O3, and NOx, an in situ estimate of &Sgr;kVOC<VOC> in the PBL is retrieved with an uncertainty of less than 20% in comparison to the detailed box model calculations. This analysis is applied to laboratory measurements with three synthetic NOx/VOC/O3 gas mixtures, and the retrieved OH lifetimes and total VOC reactivity are discussed against model predictions. ¿ 2001 American Geophysical Union